The present invention is directed to composition for use as a neutron or gamma radiation shielding material. In particular, the present invention is directed to the use of a composition consisting of SiO.sub.2 and sodium silicate in the formation of a neutron or gamma radiation shielding structure.
The problem of radiation shielding about nuclear facilities is well known. The radiation shielding provided around these facilities is often fabricated from large concrete blocks rather than monolithic concrete. The use of blocks permits reconfiguration of the shield for different experiments, and allows the shield to be disassembled for access to components located behind it. In addition, the shielding blocks are normally provided with a stepped offset to avoid direct line-of-sight radiation streaming. Because of the rather loose tolerances inherent in concrete block formation, large gaps can result between adjacent blocks. The development of a suitable radiation resistant material which can be used to fill (i.e., caulk) these gaps has been the subject of extensive research.
The material chosen as the "caulking" or radiation shielding material should be capable of functioning as either a neutron or gamma shield, or both, dependent upon the radiation source. Heavy or moderately heavy elements are usually used to attenuate the gamma radiation and to slow down very fast neutrons to about 1 MeV by inelastic collisions. Hydrogenous materials are used to moderate neutrons with energies below 1 MeV by means of elastic collisions. Materials, most notably those containing boron, are used to capture thermal neutrons without producing high energy gamma rays.
The material chosen as a radiation shielding material or "caulking" should preferably be applicable as a high viscosity paste or a liquid so as to completely fill the gaps. In addition, it should dry to a solid which is readily removable should disassembly of the shielding blocks be required. The material should also display low neutron activation characteristics, good thermal and radiation stability, and either be nonflammable or else not give off noxious or poisonous fumes during combustion.
A number of candidates for use as a radiation shielding material or "caulking material" have been considered. A list of these materials appears in Table I below, along with their potential use as either a neutron only, gamma only, or combination neutron/gamma shield.
TABLE I ______________________________________ FUNCTIONAL CAPABILITIES OF CANDIDATE RADIATION SHIELDING Material Neutron Gamma Combination ______________________________________ Lead/lead alloys x Masonite .TM. x Boral .TM. x Permali NH .TM. x Permali JN .TM. x Silicone foam x x (with lead) x (with lead) Polyethylene x x (with lead) x (with lead) Ricorad .TM. x Polystyrene x x (with lead) x (with lead) Water bags x Polyurethane x Epoxy grouts x x x ______________________________________
These materials are described in detail below.
Lead and lead alloys have been used as radiation shields predominantly because of their high density and ease of fabrication. For gamma rays with energies in the region of 2 MeV, approximately the same mass of lead as of iron is required to achieve the same degree of radiation attenuation. However, at both higher and lower energies, lead is more efficient. Because of its low melting point, lead can be used only in shields where the temperature is not too high.
Masonite.TM., a compressed wood product, with a density of about 1.3 grams per cubic centimeter and containing about 6 weight percent of hydrogen, was used as the hydrogenous material in some early reactors. It formed part of laminated shields consisting of alternate layers of Masonite.TM. and iron. The number of H.sub.2 atoms per cubic centimeter of Masonite.TM. is as high as about 5.times.10.sup.22, which is not much less than water. In addition, it contains both carbon and oxygen which can act as moderators.
A boron containing solid which may have application for neutron shielding is the complex of boron carbide (B.sub.4 C) and aluminum known as Boral.TM.. The composition of Boral.TM. is variable but it generally contains from 30 to 50 percent by weight of boron carbide, and is available in sheets, either 1/4 inch or 1/8 inch thick, clad on each side with 0.02 inch of aluminum. The product has a density of 2.5 grams per cubic centimeter; it has satisfactory mechanical properties and absorbs neutrons with no accompanying high-energy gamma radiation.
Permali NH.TM. is a special grade of densified beechwood laminate with a hydrogen content of approximately 6 percent. The beechwood veneers are impregnated under vacuum with a modified phenolic resin, and then densified through the application of heat and pressure. The densification required to produce Permali NH.TM. provides increased hydrogen concentration per unit volume. The hydrogen content is in the combined form, being a part of the hydrocarbon molecules of both constituents. Since the hydrogen is uniformly dispersed, the material is isotropic with respect to neutron shielding effectiveness. Permali NH.TM. is available in boards. For applications where neutron capture, not solely attenuation, is required, borated Permali JN.TM. is available. While maintaining a hydrogen content of approximately 6 percent, this material also contains 3 percent boron, uniformly dispersed throughout.
Silicone foam can be poured in place or sprayed in the form of an aerosol. Polyethylene film can be used as a "mold release" to assure that the caulking material can be separated from the shielding blocks. It has a typical 1 hour curing time. The maximum continuous service temperature is approximately 205.degree. C. Oxidative degradation is the primary problem. It would reach a moderate activation level, and is a good neutron shield. With lead added, it is also a good gamma shield.
Polyethylene comes in sheets or beads and it is a low activation material. It is an excellent neutron shield, especially with boron added. With the addition of lead, it is also a good gamma shield. In the beaded form, the material will completely fill the gap in the shielding blocks with about 60 percent of the solid density, which will eliminate gap streaming of neutrons and gammas. It will also provide a flexible shield. The beaded form can be vacuumed out of the gap and blown back into it. This feature will reduce the radiation exposure to personnel since they would not handle the material directly. A vacuum system and storage canisters must be provided to handle the beads. A method of sealing the beads within the gaps would also have to be provided but a material such as duct tape could probably be used.
Ricorad.TM. combines exceptionally high hydrogen concentration with high temperature resistance. There is actually more hydrogen in Ricorad.TM. than there is in water. The maximum service temperature is 177.degree. C. (350.degree. F.). Ricorad.TM. comes in sheets and it can be worked like wood. It is a low activation material and it can be borated, which makes it a good neutron shield. It is not a good gamma shield.
Polystyrene is similar to polyethylene but it has a lower hydrogen concentration. Its useful temperature range increases with radiation exposure.
The water bag concept entails the use of a flexible bag material filled with water. The water bag concept is a good neutron shield; however, the radiation resistance of the bag material is low. Also, if the bags are removed and then replaced during a maintenance operation, the probability of a leak developing is high. The water can be easily drained and replaced and the bags provide a flexible shield. Aqua-Gel.TM. is a concept similar to the water bag but the water is in a semi-solid state. It cannot be drained as with water; therefore, it would be more difficult to handle during the maintenance operation.
Polyurethane provides some of the features of polyethylene and some of those of silicone foam. It comes in sheets or can be sprayed in place in the form of an aerosol. Like silicone foam, polyethylene film can be used as a "mold release" to assure that the polyurethane foam will not adhere to the shielding blocks. The maximum continuous service temperature is 100.degree. C. It would reach a moderate activation level and as a neutron shield is intermediate between polyethylene (better) and polystyrene (worse). Its major drawback is that it produces hydrogen cyanide gas upon burning.
Conventional epoxy grouts feature moderately high hydrogen concentrations. Dependent upon the filler material used, they can be made moderately low in activation. The temperature and radiation stability can be increased by the addition of carbon black. They can be borated. Adhesion of the epoxy grout to the substrate would be the major problem. They are also expensive.
Experience with the materials listed in Table I has shown them to possess the following advantages and disadvantages:
Lead is not recommended for use as the "caulking" material for most nuclear reactor shielding applications because it is primarily a gamma shield material. It could have potential application in medical facilities or other installations using X-rays or Cobalt-60 sources. Lead is malleable, and it is conceivable that it could be deformed enough to conform to the contours and irregularities of the gaps. This is unlikely however. This problem could be overcome by melting the lead and pouring it into the joints but even this operation would not be entirely effective on joints in a horizontal plane.
Masonite.TM. is not suitable either. Unlike lead, it is not malleable and even in a 1/8 inch thickness is probably not flexible enough to conform to the contours of the gap. Moreover, it is combustible.
Boral.TM. suffers from the same problem of being a solid. The addition of boron to the "caulking" material is not warranted unless there is a high degree of thermalization achieved in the shield. This material is also extremely expensive for this application.
Permali NH.TM. would be comparable to Masonite.TM. with respect to its possible use as a "caulking" and, because of its greater thickness, would not be as flexible. Permali JN.TM.'s boration would likewise not be required unless the shield thermalized the majority of the neutrons.
Silicone foam produces a good "caulking" material. When it is applied in the form of an aerosol, there would be no problems in distributing it into either vertical or horizontal gaps. The use of polyethylene film as a "mold release" would assure its separation from the shielding blocks. It can function as either a neutron or gamma shield or both. To be used as a gamma shield, lead would be added to the formulation. It has good temperature stability and is nonflammable. It would feature a fairly high activation dose due to the silicon activation. The disadvantage of this material is the fact that the raw material itself is fairly expensive, and the foam producing equipment is also.
Polyethylene would be a very attractive material were it not for its poor thermal stability. It melts between 150-160.degree. F. and is unsuitable for use because of this.
Ricorad.TM. is an attrractive alternate to polyethylene as a neutron shield as it overcomes the service temperature problems of polyethylene. However, like the other solid sheet materials, it would be difficult to mold to the contours of the gaps. It would also be fairly expensive.
The water bag concept is not considered viable because of the high probability of leakage. Moreover, polyethylene bags would probably be used and these have the service temperature limitation.
Polystyrene is rejected for the same reason as polyethylene, i.e., low melting point. It melts at 130.degree. F. In addition, it gives off carcinogenic fumes when ignited.
Polyurethane would be attractive were it not for the fact that it gives off hydrogen cyanide gas upon burning. It has been rejected for this reason.
The conventional epoxy grouts are attractive. They fulfill the basic requirements for a "caulking" or they would not be used as grouts. With different fillers, they can be used as neutron or gamma shields or both. They have good thermal stability and can be formulated to be low in neutron activation. Their major drawback would be in finding a "mold release" agent to prevent the shielding blocks from being cemented together into a monolithic structure.
It can readily be seen that the discovery of a gamma and neutron radiation shielding material which can overcome the adhesion problems of the prior art epoxy grouts while eliminating the economic problems associated with silicone foams would be highly beneficial. Accordingly, the development of a relatively inexpensive, but effective, gamma and neutron radiation shielding material has remained a problem in the art until the discovery of the present invention.